Recently, significant attention has been focused on the shape attributes of glass sheets used in various applications, including liquid crystal display (LCD) applications. For example, glass sheets used in LCD applications should have low shape variability and low compaction. The ability to characterize the shape of a glass sheet would be a valuable process tool, but current techniques to measure the shape of glass both during and after the forming process are limited in both quality and quantity of data.
Measuring the shape of a large piece of glass, for example, a glass sheet have a one-sided surface area in excess of 9 m2, without contacting the surface, is complicated by the high optical transmission and specular reflection of the glass. Many techniques currently used for shape measurement of objects rely on diffuse scattering of light by the substrate surface, but because of the specular nature of glass, these techniques are not easily adaptable for glass sheet shape determination. Moreover, methods that rely on reflected light are generally limited to small fields of view or very large sensors that approach the size of the glass sheet being measured. These sensors typically must be positioned at a specular angle relative to the glass to function effectively. Thus, an array of sensors or tracking of a single sensor is generally needed to determine the shape of the glass sheet, increasing the cost and complexity of these systems. Furthermore, such systems are often too large to integrate into a glass forming process due to the limited space and hostile environment usually encountered.
In a conventional float process for forming glass sheets, molten glass is flowed onto and spreads across the surface of a pool, or bath, of molten tin. The process has been used for many decades, and is responsible for the ready supply of high quality glass sheet available for use in a variety of applications, and in particular window glass.
In certain applications, glass sheets are required to have characteristics (e.g. flatness, thinness, compaction, etc.) beyond those suitable for window glass. For such applications, a fusion downdraw process has been found to produce glass sheets of exceptionally high quality (e.g. surface finish) without the need for subsequent surface conditioning (e.g. grinding) that would be typical for float formed glass sheets.
In a fusion down draw process, molten glass-based material is provided to a refractory body comprising a trough. The molten material overflows the trough and flows over the refractory body in separate streams, rejoining only at the bottom of the refractory body to form a glass ribbon with pristine surfaces. The shape of the ribbon of glass during the fusion process is an early indicator of the quality attributes of the glass being manufactured, including the dimensional stability of the resultant glass sheet, and is therefore desirable to acquire.